Okay, so my current 4E group (a fighter, a two-sword ranger, a ranged rogue, a fire wizard, and a cleric) are getting ready to assassinate the high king of the most powerful kingdom on the continent. This king is known for being guarded by a crack group of soldiers, The Crimson Lion Legion. How would you suggest that I plan this encounter. Having access to a bunch of money, how do you think this king would prepare his bodyguards to resist general assassination attempts by powerful adventurers?

An epic spell that detects certain individuals (provided the king knows who is coming).

If he doesn't, perhaps a divination epic spell that reads minds as they enter the city and any minds with thoughts towards the king are flagged. This is easily stepped around by any magic item that halts such things. However, if the players aren't aware that the king is aware of their coming- might be a good way to prohibit them getting to him.

Is this an encounter or an end-game? If it's end-game then obviously you want players to be able to kill the king. If it's just another encounter then the above is redundant as they won't be able to get in without being closely watched.

I think a movie quote says it best: "No blades, no bows; leave your weapons by the door." If the PCs are unarmed, they lose a lot of their capabilities. I think - do 'hands' count as weapons in 4e?

Then the king really just needs to be protected from magical attacks. A ring with resist 15 all or some item that he always wears for magical protection... hmmm, but then the players might get it if they succeed. Maybe it only functions for the royal bloodline?

A damage-sharing spell with an advisor who is willing to lay down his life for his king. "I've done over 200 hp to this king, and he's not even bloodied?" Heh heh. Said advisor is in another castle entirely.

And, of course, ninja guards. Well, you don't want the characters disarming the guards and using their weapons, do you?

A magical effect on the palace grounds that prevents some kinds of spells from being cast properly unless you have a token of some kind. The King's pet mages would all have said token. If the PCs can manage to kill/bribe a few of these mages early on, they get the tokens and overcome it.

If the king sees the attempt coming (or is particularly paranoid) you can always go the doppelganger/fake king route. The person on the throne in the opulent robes is a decoy. The real king was the advisor who fled first round of combat.

Well, you have lots of options here. I would recommend a paladin among the guards; they have a few abilities where they block damage or take half damage. You might want a swordmage too to mark a PC and reduce the damage to the king by its aegis.

Clearly a healer to remove conditions and heal him should he be wounded.

There has got to be a few items that reduce the effectiveness of poison as well; I would include those as well.

Keep in mind that monsters don't have follow the same conventions of "class" that players do. "Swordmage" and "Paladin" are PC classes that have no bearing on the abilities of NPCs.

The first thing I think you need to do is more clearly define the encounter. ObviousTroll's comments about "end adventure" vs "end game" are very relevant. Determining the location of the encounter is important as well. An assassination attempt in the throne room of the King's main palace will play very differently than an attempt along a road somewhere. Equally important is to define the King. Is he a soldier? What kind (Brute, Soldier, Skirmisher, Artillery)? Is he a sorcerer-king? Good or evil?

The Crimson Lion Legion should:

Fight to the death. Perhaps have some form of "last attack" power when reduced to 0hp.

Include a mix of Controller, Soldier, and Skirmisher monster types. The King himself should provide Leader bonuses in addition to whatever else he can do.

Fight using excellent tactics, and perhaps have abilities to take advantage of teamwork. These guys are the pinnacle of the kingdom's military and are dedicated to the protection of the king. They are not to be trifled with!

If this is not the endgame encounter, the Lions will have a clear plan for getting the King out of the battle.

Odds are high that the King's protection detail will not be 100% Crimson Lions. Toss in some normal soldiers and minions to make dealing with the Lions that much more difficult.